Parallel generator power system

FIELD OF THE INVENTION

This invention relates to the field of power generation, and more specifically to a system for choosing between two or more electrical generators when starting a parallel generator power system.

BACKGROUND

Electrical generators or generator sets are devices which transform mechanical energy into electrical energy. A typical generator set includes an engine, driven by a fuel such as gasoline, and a rotating shaft driven by the engine. The rotating shaft develops electric power, which is then used to power an electrical load.

Sometimes, two or more generator sets are connected to a common bus in a parallel arrangement. This allows for each of the generator sets to be turned on as needed. For instance, if a light load is being fed, then only one generator set may be needed. As the load increases, a second generator set, a third generator set, and so on can be activated.

In such parallel generator set systems, a signal is sometimes received by the system to start more than one of the generator sets at a time. However, when two or more generator sets are started simultaneously, it becomes necessary for the system to determine which generator set has started first and allow that generator set to close its circuit breaker to the bus while inhibiting the other generator sets in the system from closing their breakers. This is because the generator sets cannot be simultaneously connected to the bus since their output must be synchronized in voltage, frequency, or phase before being connected to the load. Accordingly, a single generator set must be connected to the load at a time. In the past, a central control module was connected to each generator set to control the generator set connections to the bus. However, such a central controller does not allow a parallel system to be easily expanded and it can be costly to set up and maintain.

What is needed is a system and method for connecting multiple generators to a common load in which there is no central controller so that it is easier to set up a parallel generator power system and easier to add further generators onto the system without having to re-configure the whole system.

SUMMARY

Accordingly, the present invention provides a method and system for connecting multiple generator sets to a shared bus. One aspect of the present system involves a power system. In one embodiment, a power system includes two generator sets and two modules, wherein each of the two modules is associated with one of the two generator sets. Each of the two modules is configured to receive a signal from that module's associated generator set when that module's associated generator set is ready to energize the bus. Each of the two modules also includes an output to send a signal which allows that module's associated generator set to connect to the bus. The modules are connected together and configured so that when each of the modules receives the signal that the module's associated generator set is ready to energize the bus at substantially the same time, each of the two modules performs a method which provides that only one of the two modules is permitted to send the signal which allows that module's associated generator set to energize the bus.

Another aspect of the present system provides a method for connecting one of a plurality of generators to a bus. In one embodiment, a method includes providing a plurality of substantially equivalent connection modules, each of the plurality of connection modules associated with one of the plurality of generator sets, each of the plurality of connection modules configured to send a signal to its associated generator set when the connection module reaches a first-start state. The method further includes connecting each of the plurality of connection modules to its one or two neighboring connection modules to allow the connection modules to communicate with each other.

Among other advantages, the present system allows the power system to be quickly set up and easily expanded by providing separate connection control modules on each generator set instead of having a central controller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a parallel generator set power system according to one embodiment.

FIG. 2 shows details of the power system of FIG. <b>1</b>.

FIG. 3 shows a schematic diagram of electronics and hardware of a starting module of the present system.

FIG. 4 shows a state diagram of a method in accordance with one embodiment.

FIG. 5 shows a flowchart depicting an arbitration scheme according to one embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents.

FIG. 1 shows a schematic representation of a power system <b>100</b> which includes two or more engine-generator sets <b>102</b> connected in parallel to a common bus <b>110</b>. In this example, generator sets <b>1</b> and <b>2</b> are shown. The present system is applicable to almost any number of generator sets. In one embodiment, there can be between two and twenty-four generator sets connected together in parallel. Generator sets <b>102</b> feed a load <b>130</b> via bus <b>110</b>. A switch <b>104</b>, such as a circuit breaker, is located between each generator <b>102</b> and bus <b>110</b>. When a given switch <b>104</b> is closed, the associated generator set <b>102</b> is connected to bus <b>110</b> and feeds the load. A start control <b>105</b> is connected to each generator set to control the start up of the generator sets.

In one embodiment, each of generator sets <b>102</b> are diesel or gas generator sets manufactured by Cummins Power Generation, Inc. In one example use, the system is applicable to diesel generator sets in the 30 Kw to 2 Mw range equipped with hardware which is capable of automatic isolated bus paralleling.

When two or more of the generator sets <b>102</b> of system <b>100</b> are started simultaneously, it becomes necessary to determine which of the generator sets has started first and allow that generator set to close its associated circuit breaker <b>104</b> while inhibiting the other generator sets <b>102</b> from closing their circuit breakers. For instance, if generator set #<b>1</b> is started first, it must be allowed to close its circuit breaker to the bus while generator set #<b>2</b> is inhibited from closing its circuit breaker. This is because the generator sets cannot be simultaneously connected to the bus since their output must be synchronized in voltage, frequency, or phase before being connected to the load. Once one of the generator sets connects to the bus, the other generator sets recognize that the bus is energized and synchronize and close their breakers accordingly.

To provide this functionality of choosing which generator set should connect to the bus first, system <b>100</b> includes two or more connection modules <b>140</b>. Each connection module <b>140</b> is substantially equivalent with the other connection modules <b>140</b> and is associated with a separate given generator set <b>102</b>. For instance, the present system <b>100</b> shows connection modules #<b>1</b> and #<b>2</b>, each of which are associated with generator sets #<b>1</b> and #<b>2</b>, respectively. Each connection module <b>140</b> is connected to its one or two neighboring module(s) <b>140</b> by an inhibit line or interconnect line <b>120</b>. Each connection module <b>140</b> controls whether its associated generator set will receive the command to be the first generator set to connect to the bus. Since the connection modules are substantially equivalent, extra connection modules <b>140</b> and generator sets <b>102</b> can be added on to the system without having to re-configure or reprogram a central control. In one embodiment, connection modules <b>140</b> communicate with generator sets <b>102</b> via an incoming line <b>121</b> which allows the generator set to send a ready-to-load signal to the connection module, and an outgoing line <b>122</b> which allows the connection module to send a first-start signal to the generator set.

In this example, when two or more of the generator sets <b>140</b> are ready to energize bus <b>110</b> simultaneously, the connection modules <b>140</b> arbitrate or communicate via interconnect <b>120</b> for the right to send the first-start command to their generator set. In other words, the modules <b>140</b> as a whole determine which single one of the modules should allow its generator set to connect to the dead bus while inhibiting the other generator sets from connecting to the bus.

In one embodiment, as will be discussed in detail below, this arbitration process takes the form of hardware, electronics, and software in each connection module <b>140</b> which is configured so that a module <b>140</b> only allows its associated generator set to connect to the bus if the given module reaches a “first-start” state before any other modules. Moreover, as will be understood below, although each connection module <b>140</b> is functionally equivalent, only one connection module can reach the “first-start” state during the process.

Among other advantages, having separate, yet functionally equivalent, connection modules <b>140</b> associated with each of the generator sets <b>102</b> allows parallel power system <b>100</b> to be expanded by adding additional generator sets to the system in parallel and connecting the connection modules of each additional generator set in a daisy-chain manner to the connection modules of the previous generator set. There is no central control of the connection procedure so any of the modules can win the arbitrations. This de-centralized control allows for merely adding on new generators sets. No additional configuration needs to be done since each additional module can communicate and arbitrate with the existing modules.

FIG. 2 shows further details of one example of connection modules <b>140</b>. In this example, connection modules <b>140</b> are connected in an end-to-end, daisy chain manner. Each connection module <b>140</b> includes a two-way I/O <b>202</b> which connects each module to neighboring modules via inhibit line interconnect <b>120</b>.

Each connection module <b>140</b> is coupled to a power controller <b>204</b> which controls an associated generator set. Accordingly, in this example, the system includes 1, 2, . . . N power controllers <b>204</b>. In one embodiment, power controllers <b>204</b> are PCC (Power Command Control) systems manufactured by Cummins Power Generation, Inc. In one example, each power controller <b>204</b> is an integrated generator set control which provides voltage regulation, speed regulation, fault detection, voltage, current and power measurement, digital display and integrates one or more of the functions necessary to provide paralleling, such as synchronizing, load sharing control, and breaker control. One example of such a power controller is described in U.S. Pat. Nos. 5,168,208; 5,390,068; and 5,006,781, each of which are incorporated herein by reference. In some embodiments, some functions of the power controllers are incorporated into the generator set or into the connection module.

Each connection module <b>140</b> includes a plurality of phase connector inputs <b>220</b> which are coupled to an associated power controller <b>204</b>. Each module <b>140</b> also includes connections to the associated power controller <b>204</b> by a ground <b>222</b>, a battery input <b>224</b>, a first-start signal output connection <b>226</b>, and a ready-to-load signal input <b>228</b>. In one embodiment, the power source for modules <b>140</b> is delivered from a battery of the associated generator set to battery input <b>224</b>.

Ready-to-load signal input <b>228</b> receives a “ready-to-load” signal from power controller <b>204</b> when the associated generator set is up to its rated speed and voltage and is ready to energize the bus and thus ready to be connected to the bus. If connection module <b>140</b> decides (or wins an arbitration with the other connection modules) that its generator set is to be the first to connect to the bus, connection module <b>140</b> sends a “first-start” signal via output <b>210</b> to the power command member. The associated generator set is then connected to the bus and the other generator sets in the system are inhibited from connecting to the bus.

In one embodiment, each connection module <b>140</b> includes a step-down transformer which can be used by the power command member <b>204</b> to read the voltage of bus <b>110</b>. For example, a bus connector <b>230</b> can couple connection module <b>140</b> to the bus at bus connections <b>231</b>. One phase of the bus voltage can be used to inhibit operation of the first-start signal from a module <b>140</b> to its associated generator set when a bus AC voltage is present.

FIG. 3 shows a schematic view of one example of the electronics and hardware of a pair of connection modules <b>140</b>A and <b>140</b>B. In this example, each connection module <b>140</b>A and <b>140</b>B includes a microprocessor <b>310</b>A and <b>310</b>, respectively, which receive the ready-to-load signal via inputs <b>228</b>A and <b>228</b>B from their respective generator sets and the bus inhibit signal from the bus via bus connection inputs <b>231</b>A and <b>231</b>B. The microprocessors <b>310</b>A and <b>3101</b>B output a first-start signal to their associated generator set via outputs <b>226</b>A and <b>226</b>B if the module wins the arbitration and reaches a “first-start” state, as will be described below. This first-start signal is the signal which tells the associated generator set that it gets to go first and be the first generator to connect to the bus.

In one embodiment, the example connection modules <b>140</b>A and <b>140</b>B also include, respectively, a first opto <b>301</b> and <b>302</b>, a second opto <b>303</b> and <b>304</b>, and a transistor <b>305</b> and <b>306</b>. These members are coupled to the two-way inhibit line interconnect <b>120</b> and configured to allow connection modules <b>140</b>A and <b>140</b>B to communicate or arbitrate with each other via the inhibit line to determine which connection module will be allowed to send the first-start signal via output <b>226</b>A or <b>226</b>B.

For instance, in one example, when opto <b>301</b> is ON and opto <b>302</b> is OFF, then the microprocessor <b>310</b>A is programmed to know that connection module <b>140</b>A has activated the inhibit lines and no other modules have activated the inhibit lines. Conversely, when opto <b>303</b> is ON and opto <b>304</b> is OFF, then the microprocessor <b>310</b>B is programmed to know that connection module <b>140</b>B has activated the inhibit lines and no other modules have activated the inhibit lines.

The optos <b>301</b>-<b>304</b> are turned OFF and ON depending on the states of the inhibit line interconnect inputs <b>202</b> and the transistors <b>305</b> and <b>306</b>. As can be seen in FIG. 3, if transistor <b>305</b> is turned ON and transistor <b>306</b> is turned OFF, then current flows through line R<b>2</b> and opto <b>301</b> and opto <b>304</b> turn ON while optos <b>302</b> and <b>303</b> remain OFF. Then, as described above, when microprocessor <b>310</b>A sees opto <b>301</b> ON and opto <b>303</b> OFF, it knows its module has activated the inhibit lines and no other modules have activated the inhibit lines. The converse is true if transistor <b>305</b> is turned OFF and transistor <b>306</b> is turned ON. If both transistors <b>305</b> and <b>306</b> are turned ON, then all the optos <b>301</b>-<b>304</b> turn OFF and neither set has control.

Below, Table 1 shows one example implementation of the possible transistor and opto-coupler states and corresponding first-start status for the system shown in FIG. <b>3</b>.

<tables><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 1</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>Hardware States and Corresponding First Start Status</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="8"><colspec colname="1" colwidth="35pt" align="left" /><colspec colname="2" colwidth="28pt" align="left" /><colspec colname="3" colwidth="21pt" align="left" /><colspec colname="4" colwidth="28pt" align="left" /><colspec colname="5" colwidth="28pt" align="left" /><colspec colname="6" colwidth="21pt" align="left" /><colspec colname="7" colwidth="28pt" align="left" /><colspec colname="8" colwidth="28pt" align="left" /><tbody valign="top"><row><entry>Trans.</entry><entry>Trans.</entry>&lt;entry /&gt;&lt;entry /&gt;<entry>Set</entry>&lt;entry /&gt;&lt;entry /&gt;<entry>Set</entry></row><row><entry>305</entry><entry>306</entry><entry>301</entry><entry>303</entry><entry>140A</entry><entry>302</entry><entry>304</entry><entry>140B</entry></row><row><entry namest="1" nameend="8" align="center" rowsep="1" /></row><row><entry>OFF</entry><entry>OFF</entry><entry>OFF</entry><entry>OFF</entry><entry>Disable</entry><entry>OFF</entry><entry>OFF</entry><entry>Disable</entry></row><row><entry>OFF</entry><entry>ON</entry><entry>OFF</entry><entry>ON</entry><entry>Disable</entry><entry>ON</entry><entry>OFF</entry><entry>Enable</entry></row><row><entry>ON</entry><entry>OFF</entry><entry>ON</entry><entry>OFF</entry><entry>Enable</entry><entry>OFF</entry><entry>ON</entry><entry>Disable</entry></row><row><entry>ON</entry><entry>ON</entry><entry>OFF</entry><entry>OFF</entry><entry>Disable</entry><entry>OFF</entry><entry>OFF</entry><entry>Disable</entry></row><row><entry namest="1" nameend="8" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

Referring to FIG. <b>3</b> and the table above, in one embodiment each microprocessor <b>310</b>A and <b>310</b>B includes an arbitration algorithm to switch its associated transistor <b>305</b> or <b>306</b> ON and OFF in a random pattern, while the microprocessor observes the states of its associated optos. The arbitration algorithms are designed to insure that only one generator set gets the first-start command.

FIG. 4 shows a state diagram <b>400</b> depicting one example control method according to one embodiment of the present system. The control method will be described in terms of connection module <b>140</b>A of FIG. <b>3</b>. Referring to Table 1 will also help understanding of the present description.

The method is first in state <b>0</b>. The transistor <b>305</b> (the local inhibit) is turned OFF and the method goes to state <b>1</b>. In state <b>1</b>, the system goes through a random wait period. In one example, the wait period is between 1-4 msec. If the method detects that transistor <b>306</b> (the remote inhibit) is ON, the method branches to state <b>10</b>, then returns to state <b>0</b>. Alternatively, if the random wait period passes, and transistor <b>306</b> remains OFF, then the method goes to state <b>2</b>.

In state <b>2</b>, the system waits for a NO AC signal from the bus inhibit input <b>231</b> and a ready-to-load signal from the generator set via input <b>228</b>. If those two conditions are met and transistor <b>306</b> remains OFF, then method <b>400</b> begins an *arbitration loop which consists of states <b>2</b>-<b>6</b>. In the present example, the method loops through 100 cycles of states <b>2</b>-<b>6</b> with two random waits per cycle. This takes approximately 800 msec. To begin the arbitration loop, the method turns transistor <b>305</b> is ON and the method goes to state <b>3</b>. If transistor <b>306</b> remains OFF, the method branches to state <b>4</b>.

In state <b>4</b>, a random wait period is done. In this example the random wait is between 1-4 msec. Once the random wait is done, if at least one remote transistor <b>306</b> is OFF, the method branches to state <b>5</b>. If a remote transistor is ON, the method branches to state <b>10</b>. The local transistor <b>305</b> is then turned OFF and the method enters state <b>6</b>.

In state <b>6</b> another random wait is done. If the given module has gone through <b>100</b> loops (between states <b>2</b>-<b>6</b>) and if the remote inhibit is OFF, then the unit knows it has won the arbitration. The local transistor <b>305</b> turns ON and the module reaches state <b>7</b>, leaving the arbitration loop. Alternatively, if the module has not been through <b>100</b> loops, it branches to state <b>2</b> to try to complete the 100 arbitration loops. If the remote transistor is ON, it has lost the arbitration and it branches to state <b>10</b>.

From state <b>7</b>, the method sends a first-start signal to the module's generator via output <b>226</b>A (or <b>226</b>B). The method then goes to state <b>8</b>. If the generator does not switch to the bus within 1.5 seconds, the method goes to state <b>9</b>, where the first start signal is turned OFF and the local transistor <b>305</b> is turned OFF, and the method has the module go to state <b>10</b>.

In summary, control method <b>400</b> turns transistor <b>305</b> OFF and ON and observes the state of the opto couplers <b>301</b> and <b>303</b>. When the method has transistor <b>305</b> ON and opto <b>301</b> is ON and opto <b>303</b> is OFF, and the method has completed a required number of arbitration loops, then the method has the right to send a first start signal from a connection module to its generator set. In one example, method <b>400</b> turns transistor <b>305</b> ON and OFF for random time periods and for a given number of cycles. In one example, the random time periods are 1, 2, 3, or 4 msec.

The method performs the random loops to prevent the modules from turning their transistors ON at exactly the same time which would result in more than one module providing a first start signal. In one example, method <b>400</b> loops through arbitration cycles for about 800 msec and there are about 100 arbitration cycles during the 800 msec period. Accordingly, in this example, the probability of more than one first start signal is 1/(random-wait-cycle**number of cycles)=1/(4**100)=1/1.61×10**60. In other examples the number of cycles and the random wait periods can be varied to provide for different probabilities.

FIG. 5 shows a flowchart depicting an arbitration scheme <b>500</b> according to one embodiment. In general, FIG. 5 depicts what is happening in states <b>2</b>-<b>6</b> of method <b>400</b> discussed above. In <b>502</b>, the method inquires whether the remote inhibit is on. If the answer is yes, the method branches to <b>504</b>, which states that the arbitration is lost. If the remote inhibit is not on in <b>502</b>, the method branches to <b>506</b>. In <b>506</b>, the method inquires whether the loop has executed 100 times. If it has, the given module has won the arbitration and the module sends a first start signal as discussed above. Alternatively, if the loop has not executed 100 times, the method branches to <b>508</b>, where the local inhibit is activated. In <b>510</b>, a random wait is done. Then in <b>512</b>, the local inhibit is turned off, and in <b>514</b>, another random wait is done, the method then loops back to box <b>502</b>.

Thus, if a given module sees that a remote inhibit is on and the module has not activated the inhibit lines, then another module is requesting to be the first to start and other modules will drop out of the arbitration. The random waits guarantee that multiple modules will not activate and deactivate the inhibit lines simultaneously and more than one module will think they are the first to start simultaneously. Eventually all modules but one will drop out of the arbitration loop. Advantageously, the connection modules are designed so that if one unit activates the inhibit lines, that module can tell that it alone has activated the lines and that no other modules have activated the lines.

CONCLUSION

Among other advantages, the present system allows the power system to be quickly set up and easily expanded by providing separate connection controls on each generator set instead of having a central controller. Since there is no overall logic, each module has an equivalent chance of winning an arbitration, and there is no pre-determined coordinated connection procedure. This allows additional modules to be connected to the system in a daisy-chain manner without having to perform any detailed configuration.

It is understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.